In general, an electrical discharge, induced by a pulse of current between a workpiece and an electrode, results in the removal of a small crater of material from the workpiece. Electric discharge machining may be performed by the application of a plurality of electrical discharges in a highly repetitive manner. The spacing between the electrode and the workpiece, called the gap, is typically on the order of a few microns, or tens of microns, and is typically filled with a dielectric machining liquid, such as water or kerosene. The pulses of current have a nearly constant peak current value and constant pulse width having an ON state and OFF state for the predetermined machining condition. In order to induce optimal electric discharge machining, the pulse-like discharges of current must be optimally controlled in accordance with the material and hardness of the workpiece and the discharging condition at the gap.
The current waveforms for the discharges across the gap vary according to the conditions at the gap, as illustrated by graphs A to E of FIG. 2. Although the ordinate is labeled in volts, the waveforms represent the current discharged across the gap. For example, when the machining liquid decomposes into gas, tar, and carbon, and products of the discharge, or metal sludge are present in the gap, the impedance across the gap is reduced thereby resulting in a concentrated discharge as shown by graph A. This type of discharge is characterized by a fairly sharp rising edge at the beginning of the spark followed by a leveling off of the voltage. The type A discharge may be produced by an insufficient return to a dielectric state in the gap after a discharge current pulse. This may also produce a constant arcing between the electrode and the workpiece which is undesirable since it damages the electrode without contributing to the machining of the workpiece. Although the current would increase in response to a shortening in the OFF time of the pulses, contrary to the expectation that the machining rate would also be increased, the machining rate would actually be decreased due to the constant arcing.
A second type of discharge, shown by the graph B, may be created when fewer products are present across the gap. The type B discharge has a relatively favorable rising edge since it does not encounter the undesirable shaded area characterized by the arcing. The stable discharges, and hence more desirable discharges, are denoted by graphs C and D. These discharges are produced when the gap conditions are between a low level of impedance LZ and a high level of impedance HZ. The fifth type of discharge symbolized by the graph E occurs when an impurity in the gap exists resulting in a high impedance. The type E discharge does not even reach the HZ level.
When the first or the fifth types of discharges are produced, in other words those that intersect the undesirable shaded areas in FIG. 2, the OFF time of the pulses must be significantly extended or the electric machining of the workpiece will stop. Also, when a type B discharge comes close to reaching the ARC level, the OFF time must be extended in order to prevent an unstable discharge. Conversely, when type C or D discharges are produced, it is desirable to decrease the OFF time in order to increase the machining rate.
Japanese patent publication JP-B 47-50276 discloses an electric discharge machining apparatus which detects a change in the gap voltage only during the discharging time in order to control the discharge current. A first resistor is placed in series with an electrode and a workpiece for detecting the start of spark discharging. In one embodiment, a transistor responded to the start of spark discharge and to the voltage across a second resistor connected in parallel with the gap to detect gap voltage at a certain time during each pulse. In another embodiment, two transistors respond to the start of spark discharge and to the voltage across two resistors connected in parallel with the gap to detect two gap voltages at two different times during each pulse.
Japanese patent publication JP-A 47-13795 discloses another example of a conventional controller which detects a voltage variation slope, a mean voltage level, and a high frequency component. Two out of the three signals detected are then extracted to display the machining state, to control a pulse generator, or to control a servo device.
Japanese patent publication JP-A 51-119597 discloses a method of controlling pulse discharge comprised of detecting the plurality of pulses discharged across the gap; determining the machining state from the detected pulses and then producing a digital control signal. The apparatus is comprised of a discriminator for discriminating the detected pulses, a counter for counting the discriminated results, and a control signal generator for increasing or decreasing the gap distance by comparing the count to a reference level.
In order to control the machining current more accurately, it would be more preferable to detect the waveform of the discharge current. This, however, is difficult because the detecting levels, especially the level for peak current, vary significantly when the settings of the machining condition are changed to meet the machining needs.